I often see old Nikkor lenses for sale that have one deformation on the lens barrel, at the level of the filter. The focusing barrel is intact, the rest of the lens is in good condition, the glass is fine.

I can't not believe that a dent of this magnitude comes from simply dropping the lens, because I would expect some other damage, like an associated scratch on the focusing ring and the glass to have some damage!

Is there some rig people used to put their lenses on, or attach some device to the filter thread in the old film days that would cause this characteristic damage?

I'm also impressed by how much these lenses can go for - I would have thought not being able to use filters would affect the price, and the knowledge that the lens has been subject to such a large shock would cast suspicion on the lens calibration at least.